1. Field of the Invention
The present invention relates to cipher strength evaluation.
2. Description of the Related Art
The usefulness of encryption algorithms relates directly to the difficulty of decryption, which can be evaluated from the costs for the ciphertext which is actually decrypted to measure the number of plaintext and the complexity required for decryption.
In a block cipher system having a plurality of steps of accepting unstirred text, stirring with an extended key, and outputting stirred text for encrypting plaintext step by step, it prepares a decryption equation being an equation that holds on stirred text, unstirred text and extended keys, performs exhaustive search by estimating and substituting solutions, and examines whether the equation holds or not to determine the true key.
However, the system requires enormous complexity for such an exhaustive search.
Then, it utilizes an algebraic decryption method for linearizing the decryption equation being higher-order polynomial expressions, and applies Gaussian Elimination to obtained linear simultaneous equations to suppress the complexity for determining the true key.
However, a lot of plaintext is needed instead.